FIG. 11 shows a diagram of a related power supply circuit 901 for use in a series regulator. The power supply circuit 901 may be used in an LDO (Low Drop Out) regulator or the like. The power supply circuit 901 includes an output transistor 913 connected between an input terminal 911 to which an input voltage Vin is applied and an output terminal 912 from which an output voltage Vo is output. An error amplifier 914 may control the output transistor 913 based on a predetermined reference voltage Vref and a feedback voltage Vfb which may be varied based on the output voltage Vo. A reference voltage generating circuit 915 may generate the reference voltage Vref based on the input voltage Vin.
The reference voltage generating circuit 915 may be configured to generate a constant voltage. However, if the input voltage Vin as a drive voltage is varied, the reference voltage Vref may also be varied to some extent due to the variation of the input voltage Vin, which may result in an undesired variation of the output voltage Vo (see FIG. 12). As such, the variation of the input voltage Vin may deteriorate characteristics of the power supply circuit 901 of FIG. 11.